Technical Abstract:
Humic substances (HS) under conditions of adequate mineral nutrition, consistently show stimulation of plant growth in nutrient solutions. A novel screening system for HS effects on plants grown in nutrient solutions was used. Root growth was usually more apparent than stimulation of shoot growth. Typical response curves showed enhanced growth with increasing nutrient and HS concentrations, followed by a decrease in growth at high concentrations. Shoots generally showed similar trends in growth response to HS. Addition of HS was found to stimulate growth beyond that provided by mineral nutrients, presumably because of the effects of humic (HA) and fulvic (FA) acids. Plant growth experiments were carried out on turfgrasses and agricultural crops (corn and soybean). The addition of HAs or FAs alone did not result in growth enhancement or a remedy of the iron deficiency. However, addition of iron, and either HAs or FAs resulted in healthy, chlorophyll-rich plants and enhanced growth, thereby proving that improved iron availability is a major mechanism of plant growth stimulation by HS. The feasibility and impact of HS utilization in agriculture and horticulture will be presented.